Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Eternal Student on 27/02/2022 23:47:34
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Hi.
The Chemistry section never seems to get a lot of questions. So let's just ask a brief question:
Is the Chemistry of the trans-uranic elements well understood? Are they just too unstable and haven't been studied much?
For example, Carbon based Chemistry is fairly complicated and people make a living studying the chemistry just of Carbon. (I suppose you do throw in a bit of H, O and maybe some N but by and large Organic Chemisty is just about C). You have all the usual arguments about how the chemistry is diverse enough to support life. By comparsion, Silicon, doesn't seem to support anything like as many molecular structures and would be a poor choice of something to base life on. There's nothing much in the first few periods on the table that seems to support the diversity of chemistry that carbon can.
So, what about some of the really heavy elements? How well do we know their chemistry? Could there be a heavy element with Chemisry as diverse as carbon?
Also, what about this metal and non-metal stepping line anyway? Is it certain that all heavy elements behave like metals? On what basis do we decide something is a metal (other than just being on one side of the stepping line or the other). I'm mainly asking because I'm suspicious the first few answers will be that elements on the left of the stepping line don't support covalent bonds very well. How much of this is well grounded in theory as opposed to being speculation based on apparent patterns in the periodic table?
Summary: How well do we know the chemistry of the heavy elements? Could any trans-uranic elements have chemistry as complicated as Carbon?
Best Wishes.
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Trans-uranic elements are much harder to study (due to their radioactivity, rarity, and cost), but the chemistry of actinide elements (including trans-uranics) is an active field of study.
https://pubs.acs.org/doi/10.1021/acs.inorgchem.8b03603
These f-block elements don't have chemistry similar to carbon, but many of them do have quite complex chemistries. They can have many different oxidation states, and can be used in compounds that serve as catalysts (Fox, A.R.; Bart, S.C.; Meyer, K.*; Cummins, C.C.*, Towards Uranium Catalysis, Nature, 2008, 455, 341 - 349), fluorescent compounds (https://pubs.rsc.org/en/content/articlelanding/2006/cp/b607486c), and thereputics (https://www.lanl.gov/discover/publications/actinide-research-quarterly/pdfs/arq-2019-01.pdf)
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Thanks @chiralSPO .
I've started looking through this info. That's going to keep my busy for a while.
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Silicon, doesn't seem to support anything like as many molecular structures (as carbon)
Silicon chains are not very stable, but silicone chains (with an oxygen atom between each silicon atom) are quite stable. These chains are able to support a wide variety of side groups.
See: https://en.wikipedia.org/wiki/Silicone
what about some of the really heavy elements?
The chemical characteristics of an element are mainly determined by the outer shell of electrons.
- But physicists are not entirely sure that the current electron model applies in the environment of a very positive nucleus.
- For example, the periodic table suggests that element 118 (Og) should be a noble element (non-reactive).
- Theory suggests that it should be a solid at room temperature.
But with a half life of 0.7ms before it decays, it is hard to test the chemical reactivity.
- Since you only have access to 1 atom at a time, you can't test if it would form a solid at room temperature (this takes at least millions of atoms).
See: https://en.wikipedia.org/wiki/Oganesson
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Hi again.
I've enjoyed reading your links, @chiralSPO . Well, one of them was pay-walled so I haven't bothered with that much but it's sufficient to note that research papers exist on the topic. Thanks for spending the time to find some info.
But with a half life of 0.7ms before it decays, it is hard to test the chemical reactivity.
I can see the problems. Decay rates aren't immune to relativity. Can you get the radionuclides into a particle accelerator to give you more time to observe their behaviour while you remain at rest in the lab?
Is it possible to slow the decay down by other methods? A few years ago, school-level physics would have stated that nuclear reactions are unlike chemical reactions - nuclear decay is a random process and the decay rates are un-affected by environmental conditions like pressure and temperature. The general explanation being that the nucleus is dominated by the effects of nuclear forces (the strong and weak force) and effectively independent from whatever else is going on outside the nucleus. It was a good explanation but like the semolina pudding they served at school lunchtime, it was just so wrong.
This belief was seriously adjusted in my life time when a type of nuclear change called electron capture was studied. ( https://en.wikipedia.org/wiki/Electron_capture ). In electron capture it is possible to adjust the nuclear reaction rates just by ionising the atom (for example the nuclear change 74Be+ → 73Li+ proceeds more slowly than 74Be → 73Li ). You can get a smaller (but statistically significant) difference just by bonding the atom to certain things.
I've not seen any information about it - but it begs the question that you might be able to adjust the rates of other nuclear changes. For example, alpha emission may be reduced if the unstable nucleus can be surrounded with ligands that are positively charged and create a potential barrier against the emission of another positively charged particle. Maybe just putting the unstable nuceii under pressure is enough to slow the decay (since the appearance of more particles tends to increase pressure PV = nRT etc). I don't know and It's important to point out that this is just speculation: I have a personal belief that nuclear reactions are much more like chemical reactions than we had first imagined (just in terms of their reaction kinetics, obviously chemistry goes on outside the nucleus but nuclear change doesn't).
Best Wishes.
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Can you get the radionuclides into a particle accelerator to give you more time to observe their behaviour while you remain at rest in the lab?
The problem is that a particle accelerator really only works well with particles of a single mass - it would be really hard to bring Fluorine atoms into close proximity with an atom of Oganesson to see if it reacts (Fluorine is one of the few elements that reacts with noble elements).
- And even if you did, Relativity would slow down the chemical reaction rates by the same factor as it slowed down the nuclear decay.
In electron capture it is possible to adjust the nuclear reaction rates
You can reduce electron capture rates by reducing the number of electrons.
I expect that Oganesson decays by fission because the immense electric repulsive field (with infinite range) is competing with the Strong Nuclear force (which has a range similar to the radius of a Uranium nucleus).
- I vaguely recall that there has been some success maintaining metastable states in electron orbitals by probing them with lasers
- To probe a nucleus, you would need to do it with gamma rays, which raises a number of problems (does anyone have a steerable, tunable, monochromatic focused, gamma-ray source, and equivalent detectors?).
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Hi.
And even if you did, Relativity would slow down the chemical reaction rates by the same factor as it slowed down the nuclear decay.
I knew that comment was coming. I almost inlcuded it in my own post.
Everything else also looks sensible, thanks for your time @evan_au
Best Wishes.
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Is it possible to slow the decay down by other methods? A few years ago, school-level physics would have stated that nuclear reactions are unlike chemical reactions - nuclear decay is a random process and the decay rates are un-affected by environmental conditions like pressure and temperature. The general explanation being that the nucleus is dominated by the effects of nuclear forces (the strong and weak force) and effectively independent from whatever else is going on outside the nucleus. It was a good explanation but like the semolina pudding they served at school lunchtime, it was just so wrong.
This belief was seriously adjusted in my life time when a type of nuclear change called electron capture was studied. ( https://en.wikipedia.org/wiki/Electron_capture ). In electron capture it is possible to adjust the nuclear reaction rates just by ionising the atom (for example the nuclear change 74Be+ → 73Li+ proceeds more slowly than 74Be → 73Li ). You can get a smaller (but statistically significant) difference just by bonding the atom to certain things.
I've not seen any information about it - but it begs the question that you might be able to adjust the rates of other nuclear changes. For example, alpha emission may be reduced if the unstable nucleus can be surrounded with ligands that are positively charged and create a potential barrier against the emission of another positively charged particle. Maybe just putting the unstable nuceii under pressure is enough to slow the decay (since the appearance of more particles tends to increase pressure PV = nRT etc). I don't know and It's important to point out that this is just speculation: I have a personal belief that nuclear reactions are much more like chemical reactions than we had first imagined (just in terms of their reaction kinetics, obviously chemistry goes on outside the nucleus but nuclear change doesn't).
Neutrinos can also influence the rate of reverse electron capture (https://iopscience.iop.org/article/10.1086/305343/fulltext/34468.text.html)
And, obviously, nuclear fission chain reactions such as those crucial (critical?!? sorry...) to atomic bombs, show that nuclear decay can be influenced by the environment.
That said, because of the great energies typically involved in nuclear reactions, I don't think that most changes in pressures and temperatures that could be highly influential for chemical reactions would do much for nuclear ones. But the stability of neutrons in neutron stars is significantly greater than for lonely neutrons—and I think it might be due to the insane pressures within a neutron star that favor single neutrons over proton/electron pairs (and neutrinos!).
I think the case of electron capture in 74Be+ vs 74Be can be explained by looking at how much electron density is within the cross-section of the electron capture of the nucleus. However, I am not so sure that adding positive ligands around an atom would significantly reduce α decay. The electric field at the "surface" of the nucleus will be almost entirely dominated by the protons within the nucleus For example, a 210Po nucleus, which typically undergoes α-decay with a half-life of about 140 days, has 84 protons in a sphere with a radius on the order of a few femtometers—adding a handful of singly (or doubly) positively charged ligands at a radius of 150 picometers (almost 1 million times as far away) likely won't change much. In terms of the electric potential at the nucleus, I think this will be a similar story, but might have a greater effect (but, I would expect it to be the opposite of your prediction—more positive ligands leads to a more positive potential at the nucleus, making it more favorable for the nuclear charge to decrease).
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Thanks @chiralSPO .
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Thanks @chiralSPO .
you're welcome!
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adding a handful of singly (or doubly) positively charged ligands at a radius of 150 picometers
How would you get them to stay (either near to your centre atom or even to each other)?
Essentially, in order to change the rate of a nuclear reaction you have to change the energies involved by an amount comparable with the decay energy.
And the problem is that chemical energies- like ligands are about a million times smaller than typical nuclear energies.
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Hi.
Thanks @Bored chemist . It might have been more realistic to think about ligands that just offer a significant polarisation, a δ+ on the ligand etc.
I agree that the main problem is (or was) finding some external factor that rivals the size of the nuclear energy changes. As I mentioned a little earlier, when I was a schoolchild I recall being taught that nuclear reactions are completely unaffected by any physical (or chemical) conditions external to the nucleus. The decay of a nucleus was the textbook example of being as random a process as you will ever find in nature.
I don't think that's on a school syllabus any longer. At least some Nuclear reactions are not as random as we once thought. On a side note, I wonder if Schrodinger's cat thought experiment needs to be re-written. They usually have a radioactive substance decaying (or not decaying) as the random process determining if the poison is released. Are there any more good examples of a process we believe to be genuinely random left in science?
Best Wishes.
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I wonder if Schrodinger's cat thought experiment needs to be re-written.
Sean Carrol is a cat-lover, and has rewritten the Schrodinger's cat thought experiment: Instead of a radioactive decay releasing a poison gas, it releases an anesthetic gas.
- So the question becomes: "Is the cat awake or asleep?".
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Up to about element 98 they are sufficiently stable for "ordinary" laboratory chemistry, with plutonium (94) and americium (95) actually having industrial applications.
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For a given definition of "industrial", Californium (98) also has uses- as a neutron source.
Perhaps more relevantly, the colours of some of its compounds have been observed.